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C EN TRAL PROC ESSIN G UN IT
2. CENTRAL PROCESSING UNIT (CPU)
Six main registers are built into the CPU of the 740 Family.
The Program Counter (PC) is a sixteen-bit register; however, the Accumulator (A), Index
Register X (X), Index Register Y (Y), Stack Pointer (S) and Processor Status Register (PS)
are eight-bit registers.
Except for the I flag, the contents of these registers are indeterminate after a hardware
reset; therefore, initialization is required with some programs (immediately after reset the I
flag is set to "1").
7
PC
H
Fig.2.1.1 Register Configuration
2.1 Accumulator (A)
The Accumulator, an eight-bit register, is the main register of the microcomputer.
This general-purpose register is used most frequently for arithmetic operations, data transfer,
temporary memory, conditional judgments, etc.
2.2 Index Register X (X), Index Register Y (Y)
The 740 Family has an Index Register X and an Index Register Y, both of which are eight-
bit registers.
When using addressing modes which use these index registers, the address, which is added
the contents of Index Register to the address specified with operand, is accessed. These
modes are extremely effective for referencing subroutine and memory tables.
The index registers also have increment, decrement, compare, and data transfer functions;
therefore, these registers can be used as simple accumulators.
2
Index Register X (X), Index Register Y (Y)
7
A
7
X
7
Y
7
S
0
7
PC
L
7
N V T B D I Z C
0
Accumulator(A)
0
Index Register X(X)
0
Index Register Y(Y)
0
Stack Pointer(S)
0
Program Counter(PC)
Processor
0
Status Register(PS)
Carry Flag
Zero Flag
Interrupt Disable Flag
Decimal Mode Flag
Break Flag (BRK)
X Modified Operation Mode Flag
Overflow Flag
Negative Flag
Accumulator (A)
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